1. Field of the Invention
The present invention relates generally to a method for detecting input ac voltage and more specifically to a method for detecting whether the crest value of the input voltage is reaching a predetermined voltage or not.
2. Description of the Prior Art
Generally, apparatus such as electric devices are driven by commercial power sources with a voltage of 100 VAC and others, or driven through stabilized dc power sources which convert the ac voltage of commercial power source to a predetermined level dc voltage to be supplied. In that case, however, if the commercial power supply is interrupted or an instantaneous drop of the voltage, namely an instantaneous decrease of the crest value, occurs, various troubles may be caused in the electric device.
In order to solve such a problem, there has been proposed a stabilized dc power source provided with an emergency power source such as a battery which supplies a voltage for a predetermined period of time if the power supply is interrupted or other abnormality happens. By using this power source, a predetermined voltage can be supplied for a predetermined period of time; however, if the phenomenon such as the power supply interruption continues, the supplied voltage decreases to cause malfunction, breakdown, etc. of the electric device or the like. Therefore, it is desirable that the abnormality such as the aforementioned power supply interruption is rapidly detected and notified to the following electric device and others.
Apparatus, stabilized power sources, and so forth which can be utilized for both of the commercial power sources with voltages of 100 V and 200 V by automatically alternating a switch through detecting the voltage difference are also known. In this case, the level of the input voltage should be detected quickly and accurately.
Consequently, a method for the detection by using a means comprising a circuit shown in FIG. 1 (a) has been conventionally used. Namely, the voltage which is provided by the full-wave rectification of an input voltage and is supplied through a diode D1 is smoothed by a capacitor C1 connected between the terminals of an input power source, and is compared with a reference voltage E1 supplied from a reference power source 2 by a comparator 1 having hysteresis. As shown in FIG. 1 (b), when the voltage between the terminals of the capacitor C1 becomes lower than the reference voltage E1, an output of the comparator 1 varies, detecting an abnormal condition. In other words, when the power supply is interrupted, the charge stored in the capacitor C1 is discharged by a given amount and the terminal voltage of the capacitor C1 becomes smaller than the reference voltage E1, resulting in alternation of the output of the comparator 1.
However, in accordance with the conventional method described above, since the capacitor C1 is employed a large amount of current supplied by the input voltage flows into the capacitor C1, causing wave-form distortion due to the impedance of the signal source. As a result, the real crest value cannot be detected and it is difficult to determine the setting level of the reference voltage E1 serving as the reference for the abnormality detection.
Also, the charge amount in the capacitor C1 depends on the condition of the signal source, namely the crest value (instantaneous value), just before the occurrence of the abnormality such as the power supply interruption. This causes a variation of the time required for the charging, resulting in fluctuation of detection time.
Furthermore, the charge stored in the capacitor C1 is discharged in a single discharging operation when the power supply is interrupted. However, in case of the instantaneous drop where the voltage does not reach the predetermined value, the capacitor C1 is slightly charged while discharging and this charge-discharge operation is repeated alternately, decreasing the charge stored in the capacitor C1 gradually. Therefore, in the instantaneous drop it takes a longer time until the voltage between the terminals of capacitor C1 decreases to a value at which the output of the comparator 1 changes, and the detection time becomes longer than that for the power supply interruption. Thus, there is also a problem that the detection time fluctuates due to the type of the abnormal condition.